As is familiar to drivers, filling stations typically enable people to dispense fuel from a storage tank (which can be buried underground) through nozzles that are inserted into the gas tanks of vehicles. As is also familiar, the nozzles often come equipped with boots, to trap otherwise polluting hydrocarbon vapors and thereby prevent the vapors from entering the atmosphere. The trapped vapors are then returned to the tank, i.e., are recovered, by a vapor recovery system. One such vapor recover system is disclosed in the present assignee's U.S. Pat. No. 5,484,000, incorporated herein by reference.
It happens that as vapor is returned to the tank, the pressure in the tank can become greater than atmospheric pressure. When this happens, pollutants can escape from the tank through small leaks. These leaks can be difficult to detect, particularly from underground storage tanks.
Accordingly, systems have been introduced to manage pressure in filling station storage tanks. One example of such a system is disclosed in Gilbarco's U.S. Pat. No. 5,464,466, incorporated herein by reference. In the Gilbarco system, a pump recirculates vapor from a storage tank through a membrane that separates clean air from hydrocarbon vapor, with clean air being exhausted to the atmosphere and hydrocarbon vapor being returned to the tank. The pump is operated to establish a desired pressure in the tank.
The present invention recognizes that several important improvements to the art can be made. First, the present invention recognizes that many commercial embodiments of pressure management systems require large, expensive compressors and/or vacuum pumps, some of which require three phase power. Three phase power, however, is not always readily available in many locations, and the size and expense of many pumps in use, and in particular positive displacement piston-type pumps, render such systems unduly complex and expensive to procure and maintain. As recognized herein, however, it is possible to provide a pressure management system that uses simple, inexpensive, yet effective pumps.
Another problem recognized by the present invention is that when membranes are used in existing systems, the membranes can be damaged by contact with liquid that might condense in the vapor lines. However, preventing formation of liquid in the vapor lines to promote membrane operation results in nothing but hydrocarbon vapor being returned to the storage tank. We have recognized that a disadvantage of returning only vapor to the storage tank is that the vapor is lost when the storage tank is accessed to refill the tank, an occurrence that can happen as frequently as twice a day in some locations. Understandably, filling stations operators would prefer to minimize the amount of fuel they lose as vapor, and environmentalists would likewise prefer to limit the amount of hydrocarbon vapors that escape from filling stations. Fortunately, we have recognized that is possible to both return liquid to storage tanks while preventing liquid from contacting membranes in the pressure management system.
Furthermore, we have recognized that is possible for membranes and other pressure management system components to fail, potentially leading to the release of hydrocarbons to the environment through the clean air exhaust line. Unfortunately, present systems do not seem to anticipate such failure and thus do not appear to provide for warning of such failure or for corrective action for such failure. We have recognized, however, that it is possible to address this shortcoming in an efficient and cost effective way.